Wireless backhaul networks are deployed to carry the traffic between a wireless access network and the core network. For example, as described in the above referenced related co-pending applications, a wireless backhaul network may comprise a plurality of hubs, each connected to the wired core network, via Ethernet. Each hub serves multiple remote backhaul modules (RBM), in a point to multipoint configuration, using a wireless channel. Each RBM is deployed close to an access network base station, such as a small cell base station, and connected to the base station via a cable. The hubs are deployed at the locations where wired high capacity access to the core network is available, e.g. at a fiber point-of-presence.
In a wireless backhaul network, the term cluster refers to a number of RBMs and their respective serving hub. Methods and apparatus are required for determining which RBMs are served by each hub, i.e. for determining network clustering, to optimize or improve network performance. In particular, for non line-of-sight (NLOS) wireless backhaul networks it is desirable to have systems and methods for network clustering that overcome the limitations of conventional geographic location based network clustering.
Wireless backhaul networks can have different carrier frequency reuse factors. Interference is generated when multiple transmitters use the same carrier frequency. The data rate of any wireless link between a transmitter and a receiver, i.e. a hub-RBM link, in the network is directly related to the signal to interference and noise power ratio (SINR) in the system. It is desirable to have a clustering strategy that can utilize the SINR information to determine the clustering outcomes and thus to improve the average network data rate.
Hubs and RBMs are deployed at fixed locations, and hubs are located at elevated locations with sufficient height above obstacles or other environmental clutter. For example, in an urban area, hubs may be positioned on a tall building or a rooftop, above the clutter. Each RBMs is typically co-located with an access network base station, e.g. for a small cell base station, on a utility pole, sidewall of a building or other location below the roofline. Thus, typically there is not a direct line of sight between an RBM and a hub.
Based on empirical measurements, wireless channels of the backhaul network have relatively longer coherence time than those of the access network. The strongest one or two electromagnetic propagation paths between a hub and a RBM remain stationary because the environment at a sufficient height above the ground typically changes very little over time. That is, the wireless channel between any hub and RBM changes only when the environment changes significantly, such as when there is new construction, or a change of season in an area with trees. This fact motivates the use of directional antennas with fixed beam patterns for both hubs and RBMs in the backhaul network. Use of directional beam-forming antennas would be a promising enhancement to the performance for the backhaul system. In this application, the focus is on fixed beam patterns for both hubs and RBMs.
For point to multipoint coverage, any hub antenna may cover multiple RBM locations. A fixed hub pattern is used to cover a particular angle range. For example, similar to the access network, a cell, which defines the 360 degree coverage of the neighborhood around the center, can be divided into three 120 degree equally spaced sectors. In such an arrangement, each hub is deployed to cover one 120 degree sector with a fixed antenna pattern and three hub modules are co-located at the center of the cell.
An RBM antenna in the backhaul network needs to have a narrow beam width and steer its main lobe to the strongest angle of departure (AoD) direction between the hub and the RBM. Due to the narrow beam width of the RBM antenna and sufficient height of hub and RBM antennas, the power delay profile of the multi-path channel between the hub and the RBM confines most of the power in the first tap. This is different from wireless access networks where the height of mobile user antennas is low and omni-directional antennas are used for the mobile users. The frequency response of the channel in the backhaul network may have much less variability than that in the access network. The entire bandwidth may be divided into a number of frequency bins and be accessed by different hub-RBM links on a per-bin basis. The frequency domain modulation scheme can be either orthogonal frequency division multiple access (OFDMA), frequency division multiple access (FDMA) or single carrier modulation (if the number of bins is one). Based on the property of the power delay profile of the channel, the average channel gain in a pre-defined frequency bin in the frequency domain is a good representation of the channel response in this bin. The above referenced U.S. patent application Ser. No. 13/230,368, for example, discloses methods for managing resource allocation using clustering or grouping of links, based on co-channel interference metrics obtained from measurements of relative path loss or signal strength for each link.
Conventionally, in network planning, geographic location based clustering is used for wireless network clustering, i.e. using an algorithm based on the geographic position, or location index, of each transmitter and receiver module. However, such an approach may not be optimum for NLOS wireless backhaul links and is computationally complex.
For NLOS links having a similar hub-RBM separation distance, the effects of environmental clutter such as buildings or trees, referred to as shadow fading or log normal shadowing, may be quite different, and significantly affect relative path loss and signal strength. Geographic location based clustering does not account for these factors or e.g. use SINR information to improve network performance. Consequently, improved or alternative methods for network clustering are required for NLOS wireless backhaul networks.
An object of the present invention is to provide improved or alternative methods and apparatus for determining network clustering, for improved network performance in point-to-multipoint wireless networks and particularly for wireless backhaul solutions comprising fixed or stationary nodes with directional antennas, including small cell NLOS backhaul networks.